Enhanced lipid productivity of Chlamydomonas reinhardtii with combination of NaCl and CaCl2 stresses

Salinity (NaCl) stress treatment is a strategy to induce lipid accumulation in microalgae. This study aimed to investigate the effect of a combination of two salts (NaCl/CaCl2) on lipid productivity of Chlamydomonas reinhardtii. C. reinhardtii was cultured in a two-stage culture comprising 9-day active growth in C medium followed by 3-day salt stress in C medium with various concentrations of NaCl (50-200 mM)/CaCl2 (100 mM). In salt stress stage, NaCl (200 mM), CaCl2 (100 mM), and the NaCl/CaCl2 mixture inhibited growth but increased the lipid content in C. reinhardtii in comparison with NaCl (0, 50, and 100 mM) conditions. Combinatorial treatment with 100 mM NaCl/100 mM CaCl2 resulted in the highest lipid content (73.4%) and lipid productivity (10.9 mg/L/days), being 3.5- and 2.1-fold, respectively, in salt-free control conditions, and 1.8- and 1.5-folds, respectively, with 200 mM NaCl. Furthermore, 100 mM NaCl/100 mM CaCl2 treatment markedly upregulated glycerol-3-phosphate dehydrogenase (GPDH), lysophosphatidic acid acyltransferase (LPAAT), and diacylglycerol acyltransferase (DAGAT), which are involved in lipid accumulation in C. reinhardtii. The upregulation of these genes with 100 mM NaCl/100 mM CaCl2 resulted in the highest lipid content in C. reinhardtii. Therefore, stress treatment using two salts, 100 mM NaCl/100 mM CaCl2, is a potentially promising strategy to enhance lipid productivity in microalgae.